Poly(ethylene terephthalate) and its copolymers, which are commonly referred to in the industry simply as “PET,” have been widely used to make containers for carbonated soft drink, juice, water and the like due to their light weight and excellent combination of mechanical and gas barrier properties. When used for carbonated soft drink (CSD) applications, however, a higher molecular weight PET has to be used due to a phenomena called environmental stress cracking.
High molecular weight PET has a high intrinsic viscosity (IV). Traditionally, a minimum IV of 0.80 dL/g is desired in making a CSD container, with preferred IV's being reported to be 0.82 dL/g or higher. Higher IV or higher molecular weight PET has an IV above 0.80 dL/g and fewer chain ends, and is believed to have less interaction with chemical attacking agents, and therefore less stress cracking. In addition, higher IV PET is believed to have more chain entanglement, which can dissipate more stress than lower IV PET.
Higher IV PET, however, requires longer solid state polymerization (SSP) time, and higher injection molding temperature due to higher melt viscosity. The higher injection molding temperature means more degradation of PET during the injection molding and more energy consumption. The higher IV PET is also more expensive to make than lower IV PET. Nevertheless, the general trend in recent years is to use PET having an IV of about 0.84 dL/g or higher to avoid the annoyance of the environmental stress cracking problem, even though higher IV PET costs more to produce and costs more to convert from resin to container. The lower IV PET, although costing less to produce and convert, does not have enough mechanical strength. This is shown not only by the lower stress cracking resistance of the low IV PET, but also in the natural stretch ratio of the PET.
It is well known to those skilled in the art that the natural stretch ratio is an inherent property of PET. The natural stretch ratio of PET depends on the PET composition, the IV of the PET, the stretching temperature, and the stretching rate. Under the same stretching temperature and rate, the natural stretch ratio of PET decreases with an increase in the IV, or molecular weight. It is believed that the decrease is caused by the chain entanglement from the higher molecular weight PET. In container applications, the container is made from an injection blow molding process. A preform is first injection molded and then blow molded into containers either in a one step or two step process.
The natural stretch ratio determines the preform design. When designing a preform, it is important that the stretch ratio of the preform to the corresponding bottle is more than the natural stretch ratio of the polymer such that the polymer can reach and pass the strain hardening point. Only after the PET stretches past the strain hardening point will it start forming the stable orientation and strain induced crystallinity. If the preform is designed such that the polymer does not reach the strain hardening point during blow molding, or just reaches the strain hardening point, the resulting orientation and crystallinity in the bottle will be substantially lower, and the material distribution in the bottle will be more non-uniform. These properties, in turn, not only impact the mechanical properties of the bottle such as the sidewall rigidity and the thermal expansion under pressure, they also affect the gas permeation through the sidewall of the containers, and the shelf life of product stored in the containers. Therefore, it is very important that the preform is designed according to the natural stretch ratio of the polymer.
The stretch ratio as used here is the nomenclature that is well known in the art and is defined as follows:Stretch ratio=(maximum container diameter/internal preform diameter)×[(height of container below finish)/(height of preform below finish)]
Since higher IV PET has lower natural stretch ratio, a lower stretch ratio preform can be used for higher IV PET. It is also well known to those skilled in the art that the lower stretch ratio preform means a longer or bigger diameter preform with thinner sidewall under the same preform weight. The thinner sidewall means faster cooling during the injection molding (the cooling time is proportional to the square of the sidewall thickness) and shorter cycle time. For a common bottle grade low IV PET that is modified with IPA or CHDM, the preform has to have a higher stretch ratio to mold into containers such as bottles. The higher stretch ratio preform means thicker sidewalls and longer cooling time, which translate into a reduction in productivity. If a preform is designed for the high PET IV, and a low IV is used to mold such preforms, the material will not stretch properly when blowing into bottles and the sidewall crystallinity and orientation will suffer. This will cause an increase in the creep for CSD containers and a decrease in the rigidity of the bottle sidewall.
Thus, there exists a need in the art for a lower IV PET preform that has a conventional configuration normally used for high IV PET, but also is useful for making containers having suitable mechanical properties and stress cracking resistance.